A man accused of killing 11 people at a Pittsburgh synagogue during a baby naming ceremony has been charged with 29 offences.

Robert Bowers, 46, was charged with offences including hate crimes and weapons offences as he recovered in hospital having been shot several times after exchanging gunfire with police, authorities said.

The 20-minute attack at Tree of Life Congregation in Pittsburgh's Squirrel Hill neighbourhood at 10am on Saturday left six others wounded, including four police officers.

Image:Suspected gunman Robert Bowers is in custody. Pic: US media

The suspect is believed to have shouted antisemitic slurs, including "all Jews must die" as he barged into the synagogue on Saturday morning before opening fire in one of the deadliest attacks on Jews in US history.

Public safety director Wendell Hissrich described the crime scene as "horrific", saying it was one of the worst he had ever seen, while FBI special agent Bob Jones said it was the worst he had seen in 22 years of service.

Pope Francis condemned the "terrible" attack as an "inhuman act of violence" that had wounded "all of us", and prayed for the stamping out of "hotbeds of hate" - as well as for stronger moral and civil values.

Image:The mass shooting took place at the Tree of Life synagogue

Authorities said Bowers was armed with an AR-15 assault rifle - the same semi-automatic model used by last year's Las Vegas shooter - and at least three handguns when he burst into the synagogue in the state of Pennsylvania.

Scott Brady, chief federal prosecutor in western Pennsylvania, said: "The actions of Robert Bowers represent the worst of humanity. We are dedicating the entire resources of my office to this federal hate crime investigation and prosecution.

"Please know that justice in this case will be swift and it will be severe."

0:22

Video:Public safety director Wendell Hissrich says the crime scene is one of the worst he has ever seen

He said the attack was a "terrible and unspeakable act of hate".

Pittsburgh's central blood bank had queues outside it on Saturday evening while thousands of people jammed an intersection as it drizzled during a vigil for the victims.

Some chanted "vote, vote, vote" during the emotional gathering as they blamed the shooting on the nation's political climate, taking little solace in a planned visit by Donald Trump.

The US president addressed the incident on Twitter almost immediately, saying he was watching events unfold and people should remain sheltered.

He called for the death penalty for "crimes like this", as US Attorney General Jeff Sessions said federal prosecutors would seek the death penalty for the Pittsburgh shooting.

2:04

Video:'Shots fired': Moment police find synagogue gunman

A few hours later on his way to address a Future Farmers of America Convention, the president defended gun laws as he answered questions from journalists.

Mr Trump called the shooting "far more devastating than anyone thought," saying: "It's a terrible thing what's going on with hate in our country."

But he went on to say the outcome might have been different if the synagogue "had some kind of protection".

He suggested an armed guard might be a good idea for all churches and synagogues.

Israeli Prime Minister Benjamin Netanyahu said he was "heartbroken and appalled" by the attack.

"The entire people of Israel grieve with the families of the dead," he said.

"We stand together with the Jewish community of Pittsburgh. We stand together with the American people in the face of this horrendous antisemitic brutality. And we all pray for the speedy recovery of the wounded."

Image:SWAT members at the scene

World Jewish Congress President Ronald S Lauder called the shooting "an attack not just on the Jewish community, but on America as a whole".

The synagogue is a fortress-like concrete building, its facade punctuated by rows of swirling, modernistic stained-glass windows illustrating the story of creation, the acceptance of God's law, the "life cycle" and "how human-beings should care for the earth and one another", according to its website.

At the farmers convention, Mr Trump began by addressing the shooting.

Image:A vigil takes place in Pittsburgh after the shooting

"As you know, earlier today there was a horrific shooting targeting and killing Jewish Americans," he said.

"This wicked act of mass murder is pure evil, hard to believe and frankly something that is unimaginable.

"Our nation and the world are shocked and stunned by the grief.

"This was an antisemitic act.

"You wouldn't think this is possible in this day and age.

"Our minds cannot comprehended the cruel hate and the twisted malice that could cause a person to unleash such terrible malice during a baby naming ceremony."

He called antisemitism "vile and hate-filled" and said there must be no tolerance for any form of religious hatred.